1. Technical Field
This invention relates generally to a method and apparatus for controlling homogeneous-charge compression-ignition (HCCI) engines, and more particularly to a method and apparatus for controlling the combustion timing in HCCI engines by altering the reactivity of fuel, or of a mixture of air and fuel, used in HCCI combustion.
2. Background Art
Homogeneous-charge compression ignition (HCCI) is a mode of combustion in which a premixed air/fuel charge is elevated in temperature during the compression stroke until the thermodynamic conditions of the premixed charge are satisfactory for the initiation of combustion. HCCI combustion occurs when a lean homogeneous mixture of fuel and air begins combustion toward the end of the engine compression stroke. A homogeneous mixture of fuel and air can be created using conventional automotive-style port fuel injectors or by early, i.e., near bottom dead center (BDC), direct in-cylinder fuel injection. The thermodynamic conditions and temperature-time history of the mixture must be correct for pre-flame reactions to occur. HCCI combustion is characterized by multiple combustion sites in a lean fuel charge so that the peak flame temperature is similar to the bulk gas temperature. Low peak flame temperature, relative to conventional diesel diffusion flame combustion, results in NO, emissions that are 90% to 98% lower than typical conventional diesel combustion emissions.
HCCI combustion is difficult to control, primarily due to an inability to accurately and repeatably control the start of combustion (SOC) for each compression stroke. If conditions are not favorable for combustion, misfire will occur. For example, if the start of combustion occurs too early, the mixture will burn during the compression stroke, resulting in possible engine damage and poor cycle efficiency. Also, early SOC contributes to high gas temperatures in the next engine cycle, which will again lead to early SOC. Typically, exhaust gas recirculation. (EGR) is used in an HCCI engine to raise the intake gas temperature to a level where HCCI combustion will occur.
However, combustion phasing through intake gas temperature control has several inherent disadvantages. First, air intake temperature provides weak control, i.e., a large change in temperature results in only a modest change in combustion phasing. Secondly, high intake air temperatures result in decreased volumetric efficiency and reduced power output. Thirdly, even with complete control of intake air temperature, there is only limited control of the reaction phasing. Also, if recirculated exhaust gas is used for temperature control, the exhaust gas is a diluent that can also effect combustion rate. Moreover, controlling intake air temperature is difficult in practical engines. In homogeneous-charge compression engines, the inability to effectively control combustion timing results in decreased power output and efficiency.
U.S. Pat. No. 5,832,880 granted Nov. 10, 1998 to Daniel W. Dickey for an APPARATUS AND METHOD FOR CONTROLLING HOMOGENEOUS CHARGE COMPRESSION IGNITION COMBUSTION IS DIESEL ENGINES, and assigned to the assignee of the present invention, describes an apparatus and method for controlling homogenous-charge compression-ignition combustion by water injection and thereby regulating the start of combustion during the compression stroke, or early in the expansion stroke, of each combustion cycle. Although the apparatus and method described in the Dickey patent can control the start of combustion in HCCI engines, they require an on-board supply of water, a water pump, a water flow distribution system, and additional, or dual-fluid, injectors to provide both water and fuel to the combustion chamber of the engine. These requirements add to the cost and control complexity of an HCCI diesel engine.
The present invention is directed to overcoming the problems set forth above with respect to current methods of controlling HCCI combustion. It is desirable to have a method and apparatus for controlling HCCI combustion that does not rely on intake temperature control or water injection. It is also desirable to have such a method and apparatus that effectively controls HCCI combustion without sacrificing power output and engine efficiency, or adding significant cost to the engine.
In accordance with one aspect of the present invention, a method for controlling combustion timing in an homogenous-charge compression-ignition engine includes providing an intake manifold that extends between a source of air and an intake port of the engine and has a flow-through portion and a by-pass portion. The by-pass portion has a catalyst incorporated therein that is adapted to oxidize a mixture of air and fuel passing through the by-pass portion. The method further includes providing a source of fuel that is in fluid communication with the intake manifold at a position between the source of air and the by-pass portion of the intake manifold. A continuous flow of air is introduced into the intake manifold. Fuel is injected into the continuous flow of air and forms a mixture of air and fuel in the intake manifold. A means is provided for controlling the relative amounts of the mixture of air and fuel passing through the flow-through and by-pass portions of the intake manifold. At least one engine operating parameter is sensed and the relative amounts of the mixture of air and fuel passing through the flow-through and by-pass portions of the intake manifold are controlled in response to the sensed value of the engine operating parameter.
In accordance with another aspect of the present invention, a method for controlling the combustion timing in an homogenous-charge compression-ignition engine includes providing a fuel conduit that extends from a source of fuel to at least one fuel introduction port that is in fluid communication with a combustion chamber of the engine. The fuel conduit has a flow-through portion and a by-pass portion. The by-pass portion has a means incorporated therein that is adapted to decompose fuel passing therethrough. The method further includes providing a means for controlling the relative amounts of fuel passing through the flow-through and by-pass portions of the fuel conduit. At least one engine operating parameter is sensed and the relative amounts of fuel passing through the flow-through and bypass portions of the fuel conduit is controlled in response to a sensed value of the engine operating parameter.
In accordance with yet another aspect of the method for controlling the combustion timing in an homogenous-charge compression-ignition engine includes providing a fuel conduit that extends between a source of fuel and at least one fuel introduction port that is in fluid communication with a combustion chamber of the engine. The method further includes providing at least one source of a fuel additive adapted to alter the reactivity of the fuel. The source of fuel additive is in controllable fluid communication with the fuel conduit. The amount of fuel additive introduced into the fuel conduit is controlled in response to a sensed value of at least one engine operating parameter.
In accordance with still another aspect of the present invention, an apparatus for controlling the combustion timing of an homogenous-charge compress-ignition engine has an intake port through which a mixture of air and fuel is introduced into the combustion chamber of the engine. The apparatus includes an intake manifold extending between a source of air and the intake port of the engine. The intake manifold has a flow-through portion and a by-pass portion. A source of fuel is disposed in controlled fluid communication with the intake manifold at a position between the source of air and the by-pass portion whereat fuel injected into the intake manifold forms a mixture of air and fuel. A catalyst is disposed in the by-pass portion of the intake manifold and is adapted to oxidize a mixture of fuel and air passing through the by-pass portion. The apparatus further includes a means for controlling the relative amounts of the mixture of air and fuel passing through the flow-through and by-pass portions of the intake manifold, and a means for sensing at least one engine operating parameter and providing a signal correlative of the sensed engine operating parameter. The apparatus also includes an electronic control unit in electrical communication with the means for controlling the relative amounts of the mixture of air and fuel passing through the flow-through and by-pass portions of the intake manifold and with the means for sensing at least one engine operating parameter and providing a signal correlative of the sensed engine operating parameter. The electronic control unit is adapted to provide a control signal to the means for controlling the relative amounts of the mixture fuel and air passing through the flow-through and by-pass portions of the intake manifold in response to the signal correlative of the sensed engine operating parameter.
In still another aspect of the present invention, an apparatus for controlling the combustion timing of an homogenous-charge compression-ignition engine having at least one fuel injector in direct fluid communication with a combustion chamber of the engine includes a fuel conduit that extends between a source of fuel and the fuel injector. The fuel conduit has a flow-through portion and a by-pass portion. The by-pass portion has a means incorporated therein that is adapted to decompose fuel passing therethrough. The apparatus further includes a means for controlling the relative amounts of fuel passing through the flow-through and by-pass portions of the fuel conduit and a means for sensing at least one engine operating parameter and providing a signal correlative of the sensed engine operating parameter. The apparatus also includes an electronic control unit in electrical communication with the means for controlling the relative amounts of fuel passing through the flow-through and by-pass portions of the fuel conduit and with the means for sensing at least one engine operating parameter. The electronic control unit provides a control signal to the means for controlling the relative amounts of fuel passing through the flow-through and by-pass portions of the fuel conduit in response to the signal correlative of a sensed engine operating parameter.
In accordance with an additional aspect of the present invention, an apparatus for controlling the combustion timing of an homogenous-charge compression-ignition engine having a first conduit extending between a source of fuel and at least one fuel introduction port includes a source of at least one fuel additive that has a composition sufficient to alter the reactivity of the fuel when admixed therewith. The apparatus further includes a second conduit in controlled fluid communication with a source of the fuel additive and the fuel conduit. A means is provided for controlling the amount of fuel additive passing through the second conduit. An additional means is provided for sensing at least one engine operating parameter and providing a signal correlative of the sensed engine operating parameter. The apparatus further includes an electronic control unit in electrical communication with the means for controlling the amount of fuel additive passing through the second conduit and with the means for sensing at least one engine operating parameter and providing a signal correlative of the sensed operating parameter. The electronic control unit is adapted to provide a control signal to the means for controlling the amount of fuel passing through the second fuel conduit in response to the signal correlative of the sensed engine operating parameter.